random: document get_random_int() family

random: document get_random_int() family

commit 92e507d216139b356a375afbda2824e85235e748 upstream.

Explain what these functions are for and when they offer
an advantage over get_random_bytes().

(We still need documentation on rng_is_initialized(), the
random_ready_callback system, and early boot in general.)

Signed-off-by: George Spelvin <lkml@sdf.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>